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First ground-based test observations with GREAT completed successfully

It has been an exciting few weeks conducting the first ground-based astronomical test observations, referred to as 'Line Ops', with the German Receiver for Astronomy at Terahertz Frequencies (GREAT), at the SOFIA operations centre at Palmdale in the Mojave Desert. The mission was to install GREAT and carry out the initial operational tests.

A few critical actions had to be performed to transport GREAT to the telescope flange, where the instrument connects to the telescope. First, it was positioned on a platform that would be placed on a scissor lift. The platform was then lifted up to the narrow door of the Boeing 747 SP. From there, GREAT reached the aircraft interior by passing through a specially-built ramp that had previously been aligned down to the last millimetre. GREAT was turned to the right and moved over the reinforced floor of the side gangway to the telescope flange in the rear part of the plane.

The GREAT instrument (left) is attached to the telescope flange (blue, in the background)

Rolf Güsten, the PI for GREAT, carefully but calmly watched every part of the action. The GREAT team, supported by some of the technicians from the Dryden Aircraft Operations Facility (DAOF), were perfectly coordinated and had previously carried out tests in the laboratory to ensure the instrument passed through the door. Everything went smoothly; the half-ton instrument was mounted on the flange of the SOFIA telescope by early afternoon.

On Saturday 22 January, the entire telescope had to be rebalanced due to the additional load. The telescope team from the German SOFIA Institute (DSI) carried out the routine fine balancing process. They were able to determine GREAT's centre of gravity down to just a few centimetres. The optical alignment of the instrument was carried out by first positioning a liquid nitrogen-cooled test surface in front of the telescope's secondary mirror, and then observing it with GREAT.

Monday, 24 January was the day of the all-important EMI test, for which the plane was towed out of the hangar. The flight radar and the Pratt and Whitney JT9D-7A engines (also a German contribution) were started. The test was performed to determine whether GREAT's electronics disturbed the aircraft systems, and if it experienced interference from the flight systems. After all, GREAT is a heterodyne receiver: it observes signals between 1.25 – 1.5 terahertz (THz) (first low frequency channel) and 1.8 – 1.92 THz (second low frequency channel). There is also a mid-frequency channel (2.4 – 2.7 THz) and a high frequency channel (around 4.7 THz).

SOFIA was towed out of the hangar for the ground test and aligned with a target (the Pole Star)

The receiver must first convert the high frequency (HF) astronomical signals (240 to 60 microns or 1.5 to 4.7 THz) to intermediate frequency (IF) signals. To do this, it mixes the incoming signals with the output of a local oscillator (LO) that provides a stable, monochromatic reference signal. The superconducting mixers work at temperatures of 4.2 Kelvin (-269 degrees Celsius) and must be cooled using liquid nitrogen and liquid helium. The IF signal is amplified, frequency-shifted again and then analysed by a Fast Fourier Transform Spectrometer (FFTS). GREAT is extremely sensitive to external interference – from mobile phones for example. The test resulted in an 'all clear', giving a green light for actual line ops.

From Monday 25 January to Tuesday 1 February, we spent a total of four nights performing astronomical test observations outside the hangar. After the usual crew briefings, all teams gave their "Ready to go!" signal. While the winter in the north and east of the US set in with catastrophic predictions of 'monster storms', conditions in Palmdale were excellent, despite the obvious drop in temperature and snow-covered San Gabriel Mountains. In any case, clear, starry skies were forecast for the following few nights.

The aircraft was towed out of the hangar for line ops every day and positioned (or, technically, headed) so that the region around the North Star was in the line of sight when the telescope door was opened. SOFIA had to face East, as the telescope door is on the port of the aircraft. Because southern California lies at a low latitude (Los Angeles lies at 34 degrees, 3 minutes north), the Pole Star was relatively low in the sky. In Bonn, which is further north, the star has an elevation of 50 degrees, 44 minutes. SOFIA can observe the Pole Star from both of these locations, as the telescope mount's elevation ranges from 20 to 70 degrees. Observing the Pole Star would be difficult if the aircraft was flying over the North Pole because the star's elevation angle is almost vertical. This would force the plane to bank (turn on its roll axis) at 50 degrees (the telescope's nominal observation angle is 40 degrees), giving the crew reason to complain.

Inside SOFIA during the ground test

SOFIA flies at high altitudes to perform observations not affected by absorption by water vapour. Due to this, line ops had to take into account the fact that GREAT is completely blind on the ground.

GREAT was aligned with the telescope's optical axis so that the observations could be simulated using the SOFIA telescope's optical camera instead of the absent Terahertz signal. The team used the cameras to locate the Pole Star and observe a nine-point grid, for example, with predetermined relative offsets. The aim was to test the interaction between the KOSMA-I/O software and the Mission Command and Control System (MCCS) via 'Translator' – a task that kept Jürgen Stutzki, Stefan Heyminck, Urs Graf and Sean Colgan (NASA Ames) extremely busy. The atmosphere during on-board testing was professional and calm, the commands were brief and concise – a hand signal was often sufficient. The team even put on their MADS headsets for a longer observation sequence during a simulated flight segment or 'leg' on the final night. By the end of the night, Mission Operations Manager, Charlie Kaminski, and the Test Director, Walter Miller were totally convinced of the GREAT team’s professionalism.

On Wednesday, 2 February, GREAT was uninstalled - the bolts were carefully removed from the equipment flange and GREAT was brought back to the laboratory safely. In the future, this will become a routine procedure for the observatory, but with a variety of astronomical instruments (currently there are eight to choose from) designed for different scientific observations.

We can now positively look forward to the next line ops, scheduled for late February, and to the scientific flights with GREAT, planned for April.

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Dietmar Lilienthal has been a member of the SOFIA project for 15 years and he supervised the electrical and software integration of the SOFIA telescope into the Boeing 747SP in the United States. He also created the contractual undertakings for the establishment of the German SOFIA Institute (DSI) at the University of Stuttgart.
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